The present invention relates to a type of composite material which includes fiber material as reinforcing material embedded in a mass of matrix metal, and more particularly relates to such a type of composite material in which the reinforcing material is an alumina-silica fiber material including a significant amount of the mullite crystalline form, and the matrix metal is aluminum, magnesium, copper, zinc, lead, tin, or an alloy having one or more of these as principal component or components.
In the prior art, relatively low melting point metals such as aluminum, magnesium, copper, zinc, lead, tin, or alloys having one or more of these as principal component or components have been very popular for use as materials for elements which are in sliding contact with mating members, because of their affinity for such mating members and their good frictional characteristics. However nowadays, along with increasing demands for higher mechanical performance, the conditions in which such materials are required to operate are becoming more and more harsh, and tribological problems such as excessive frictional wear and adhesion burning occur more and more often; in the extreme case, these problems can lead to seizure of a moving element. For instance, if a diesel engine with aluminum alloy pistons is run under extreme conditions, there may arise problems with regard to abnormal wear to the piston ring grooves on the piston, or with regard to burning of the piston and of the piston rings.
One effective means that has been adopted for overcoming these tribological problems has been to reinforce such a relatively low melting point metal or alloy by an admixture of reinforcing fibrous material made of some extremely hard material. Thus, various composite materials including fibrous materials of various kinds as reinforcing material have been proposed. The advantages of such fiber reinforced materials include improved lightness, improved strength, enhanced wear characteristics, improved resistance to heat and burning, and so on. In particular, such concepts are disclosed in Japanese Patent Laying Open Publications Nos. Sho 58-93948 (1983), Sho 58-93837 (1983), Sho 58-93841 (1983), and Sho 59-70736 (1984), of all of which Japanese patent applications the applicant was the same entity as the assignee of the present patent application, and none of which is it intended hereby to admit as prior art to the present application except insofar as otherwise obliged by law. Further, for the fiber reinforcing material, there have been proposed the following kinds of inorganic fiber materials: alumina fiber, alumina-silica fiber, silicon carbide fiber, silicon nitride fiber, carbon fiber, potassium titanate fiber, and mineral fibers; and for the matrix metal, aluminum alloy and various other alloys have been suggested. Such prior art composite materials are disclosed, for example, in the above cited Japanese Patent Laying Open Publications Nos. Sho 58-93837 (1983) and Sho 58-93841 (1983).
However, in the case of using alumina fibers as the reinforcing material for a composite material, the problem arises that these alumina fibers are very expensive, and hence high cost for the resulting composite material is inevitable. This cost problem, in fact is one of the biggest current obstacles to the practical application of certain composite materials for making many types of actual components. On the other hand, in contrast to the above mentioned alumina fibers, mineral fibers whose principal components are alumina and silica are very inexpensive, and have conventionally for example been used in quantity as heat insulation fibers, in which case they are used in the amorphous crystalline form; therefore, if such fibers could satisfactorily be used as reinforcing fiber material for a composite material, then the cost could be very much reduced. However, the hardness of alumina-silica fibers is substantially less than that of alumina fibers, so that it is easy for the wear resistance of such a composite material to fall short of the optimum. Further, with these types of fibers used as reinforcing fiber material, since alumina-silica fibers, and particularly alumina-silica fibers in the amorphous crystalline phase, are structurally unstable, the problem tends to arise, during manufacture of the composite material, either that the wettability of the reinforcing fibers with respect to the molten matrix metal is poor, or alternatively, when the reinforcing alumina-silica fibers are well wetted by the molten matrix metal, that a reaction between them tends to deteriorate said reinforcing fibers. This can in the worst case so deteriorate the strength of the resulting composite material that unacceptable weakness results. This problem particularly tends to occur when the metal used as the matrix metal is one which has a strong tendency to form oxides, such as for example magnesium alloy.
In this connection, hardness in a resulting composite material is also a very desirable characteristic, and in the case that the reinforcing fiber material is relatively expensive alumina fiber material the question arises as to what crystalline structure for the alumina fiber material is desirable. Alumina has various crystalline structure, and the hard crystalline structures include the delta phase, the gamma phase, and the alpha phase. Alumina fibers including these crystalline structures include "Saffil RF" (this is a trademark) alumina fibers made by ICI of the U.K., "Sumitomo" alumina fibers made by Sumitomo Kagaku KK, and "Fiber FP" (this is another trademark) alumina fibers made by Dupont of the U.S.A, which are about 100% alpha alumina. With the use of these types of reinforcing fibers the strength of the composite material becomes very good, but since these fibers are very hard, if a member made out of composite material including them as reinforcing material is in frictional rubbing contact with a mating member, then the wear amount on the mating member will be increased. On the other hand, a composite material in which the reinforcing fiber material is alumina fibers with a content of from 5% to 60% by weight of alpha alumina fibers, such as are discussed in the above cited Japanese Patent Laying Open Publication No. Sho 58-93841 (1983), has in itself superior wear resistance, and also has superior frictional characteristics with regard to wear on a mating member, but falls short in the matter of hardness.